M. Heydari-Malayeri - Paris Observatory

A member of a small class of → X-ray pulsars with long rotation
periods (6-12 seconds), short → spin-down
times (~ 103-105 years), and
→ soft X-ray spectrum. AXPs show no evidence of
being → X-ray binary systems.
Their magnetic fields, as deduced from their spin-down rate, are the highest known,
reaching 1013-1015 → gauss.
AXPs are generally believed to be → magnetars.

An astronomy satellite launched by NASA in 1999 July,
specially designed to detect X-ray emission from very hot regions of
the Universe such as exploded stars, clusters of galaxies, and matter around black holes.
Chandra carries a high resolution mirror (aperture 1.2 m, focal length 10 m),
two imaging detectors (HRC and ACIS),
and two sets of transmission grating spectrometer (LETG and HETG). Important Chandra
features are: an order of magnitude improvement in spatial resolution,
good sensitivity from 0.1 to 10 keV, and the capability for high
spectral resolution observations over most of this range.
Chandra was initially given an expected lifetime of 5 years, but
on 4 September 2001 NASA extended its lifetime to 10 years "based on
the observatory's outstanding results." Among the results obtained using
Chandra one can mention the spectacular image of the
→ supernova remnant Cassiopeia A. See also
→ X-ray astronomy.

The short wavelength, high energy end of the electromagnetic
spectrum. Hard X-rays are typically those with energies greater than
around 10 keV. The dividing line between hard and soft X-rays is not
well defined and can depend on the context.

A member of one of the two main classes of → X-ray binary
systems where one of the components is a neutron star or a black hole and the other one
a → massive star. HMXBs emit relatively
→ hard X-rays and usually show regular pulsations,
no X-ray bursts, and often X-ray eclipses. Their X-ray luminosity is much larger
than their optical luminosity. In our Galaxy HMXBs are found predominantly in the
→ spiral arms and within the
→ Galactic disk in young
→ stellar populations less than 107 years old.
One of the most famous HMXB is Cygnus X-1 which was the first stellar-mass black hole
discovered. See also: → low-mass X-ray binary.

An X-ray source that is not in the nucleus of a galaxy, and
is more luminous than 1039 ergs s-1, brighter than the
→ Eddington luminosity of a 10
→ solar mass → black hole.
In general, there is about one ULX per galaxy in galaxies
which host ULXs. The Milky Way contains no such objects.
ULXs are thought to be powered by → accretion
onto a → compact object.
Possible explanations include accretion onto → neutron stars
with strong → magnetic fields,
onto → stellar black holes (of
up to 20 → solar masses)
at or in excess of the classical Eddington
limit, or onto → intermediate-mass black holes
(103-105 solar masses).
NGC 1313X-1, NGC 5408X-1, and NGC 6946X-1 are three ULXs with X-ray luminosities
up to ~ 1040 erg s-1
(Ciro Pinto et al., 2016, Nature 533, N) 7601).

The → electromagnetic radiation with
→ wavelengths shorter than that of
→ ultraviolet radiation and greater than that of
→ gamma rays.
Typical X-rays have a wavelength ranging from 0.1 to 100 Å
(0.01 to 10 → nanometers),
corresponding to frequencies in the range
3 × 1016 to 3 × 1019 Hz and energies in the range 100 eV to 100
→ keV.
X-rays are produced artificially when high-speed → electrons
collide with a heavy metal target such as tungsten. Astrophysical sources of X-rays
include → plasmas with → temperatures
in the range 106-108 K,
and deceleration process of rapidly moving charges upon interaction with matter
(→ bremsstrahlung).
X-rays were discovered in 1895 by Wilhelm Röntgen, a German physics
professor (→ roentgen).
For this discovery he won the first Nobel prize in physics in 1901.
See also: → soft X-rays,
→ hard X-rays.

X stand for "unknown," since Röntgen was not sure what they were;
→ ray.

X-ray astronomy

اخترشناسی ِ پرتوهای ِ ایکس

axtaršenâsi-ye partowhâ-ye iks (#)

Fr.: astronomie en rayons X

The study of celestial bodies using their X-ray emission. X-ray astronomy deals
mainly with Galactic and extragalactic phenomena involving very high-energy
photon emissions, covering a band of energies between 0.1 keV and 500 keV.
The research field includes:
→ X-ray binaries,
→ cataclysmic variables,
→ pulsars,
→ black holes,
→ dark matter,
→ active galaxies,
→ galactic clusters
→ X-ray transients.
The Earth's atmosphere absorbs most X-rays coming from outer space. X-ray astronomy
therefore requires observations to be done above atmosphere. The first rocket
flight which successfully detected a cosmic source of X-ray emission was launched
in 1962 by an American research group. A very bright source was detected that they named
→ Scorpius X-1.
Since then several dedicated X-ray astronomy satellites have been launched,
among which: Uhuru, INTEGRAL, ROSAT,
Rossi X-ray Timing Explorer (RXTE),
→ Chandra X-ray Observatory, and
→ XMM-Newton, which have contributed to important advances
in astronomy.

A diffuse background radiation in X-ray wavelengths which has several origins.
At very low energies it is due to hot gas in the → Local Bubble.
In the → soft X-ray energy band it comes from active
galaxies at moderate redshifts. In → hard X-ray
range the background is thought to be due to integrated emission from many
→ quasars at various redshifts.

A binary star system where one of the stars has evolved and collapsed into an extremely
dense body such as a → white dwarf,
a → neutron star, or a → black hole.
The enormous gravitational attraction of the massive, dense, but dim component
pulls material from the
brighter, less massive star in an → accretion disk.
The gravitational potential energy of the accreted matter is converted to heat
by → viscosity and eventually to high-energy photons
in the X-ray range. The brightest X-ray binary is
→ Scorpius X-1.

A rapid and intense surge of X-ray emission from some
sources. They often last less than one second followed by an
exponential decrease of typically a few seconds to a minute. Most X-ray
bursts are believed to arise in → X-ray binary
systems due to nuclear fusion of material accreted onto a compact companion.

The diffraction of X-rays by the atoms or ions of a crystal.
The wavelength of X-rays are comparable to the size of interatomic
spacings in solids. Since the atoms in a crystal are arranged in a set of regular
planes, crystals serve as three-dimensional diffraction gratings for X-rays.
Planes of repetition within the atomic structure of the mineral diffract the X-rays.
The pattern of diffraction thus obtained is therefore used to identify
minerals by bombarding them with X-rays.

A member of a class of isolated, radio-silent → pulsars
with peculiar properties. They show
a purely thermal spectrum at X-ray
energies with no evidence for a high-energy, power-law component often
detected in other → isolated neutron star
classes. The X-ray luminosity is 1031 -
1032 erg s-1, fully consistent with surface blackbody emission
with temperatures ~ 40-100 eV and (radiation) radii of a few kilometers, as
derived from X-ray spectral fits. With the only exception of RX J1856.5-3754,
broad absorption features have been found in all XDINSs. These features have
energies ~ 300 - 700 eV, equivalent widths of ~ 50 - 150 eV and, as in
the case of RX J0720.4-3125, may be variable.

A regularly variable X-ray source in which the pulsation is associated
with the rotation of a magnetized neutron star in an
→ X-ray binary. Periods range from a few seconds to a few
minutes. Examples include Hercules X-1, Centaurus X-3, Cygnus X-3.

An astronomical object whose dominant mechanism of radiation is through X-ray emission.
X-ray sources contain an extremely hot gas at temperatures from
106 to 108 K. They are generated by various physical processes
involving high energies, such as accretion on to a compact object, shock waves from
supernovae, stellar winds, hot gas in stellar coronae, or hot spaces between galaxies
in a cluster. The first celestial X-ray source, after the Sun, to be detected was
→ Scorpius X-1 by means of rocket flight (Giacconi et al. 1962).